Thank you for the additional input.
I worked some more on all of this earlier this evening. My results were similar to yesterday's.
I found that I could exhale to a certain degree then reach air, but it was inconsistent. If I held my breath, it was easier to reach air. I experimented with how much air to expel. The more I expelled, the less likely it was that I would reach air.
Not finding air is an uncomfortable feeling, needless to say. I hope I can establish good breathing, balance, and swimming skills, so I can continue to progress.
Re overrotation: I don't think my shoulders were stacked. If anything, I was rolling to my back some of the time. But I'll continue to keep tabs on this.
I did catch myself sometimes pushing my head up to try to reach air.
Here is another bad habit I caught myself doing, unrelated to breathing. When working on zipperswitch, I caught myself forcefully plunging my recovery arm ahead of me, with more tension and effort than is probably needed. I first noticed this when I felt that my shoulder was sore from the effort. I'll work on this in upcoming days.
Another question occurred to me while working on all of this. How does one transition from floating in the water to a good balanced swimming position, without pushing off from the wall, floor, etc.? I found that I couldn't get into good form if I couldn't push off from something.